This invention relates to microwave filters.
A spur-line filter is constructed of microstrip transmission lines. Conventional spur-line filter is a 50-ohm line with uniform and symmetric in-line etching. However, this technique has very narrow design tolerance. It is often very difficult to achieve an optimum design.
A conventional microstrip filter has a structure as shown in FIG. 1(a). The bottom part is a grounded metallic layer 1. Above the metallic layer 1 is a dielectric plate 2 with a thickness h and a relative dielectric constant .epsilon..sub.r. On top of the dielectric plate 2 is a microstrip line 3 with length L and a width W. FIG. 1(b) shows the transmission characteristic of a band-stop filter, showing the insertion loss (S21) as a function of frequency. The insertion loss transmission coefficient S21 can be defined as the ratio of the filter output b.sub.2 and the filter input a.sub.1, expressed in dB as 20 log(b.sub.2 /a.sub.1) (dB). Therefore 0 dB indicates zero loss and ordinary insertion loss of a passive filter has a negative dB value. An ideal filter has maximum loss in the stop-band, minimum loss in the pass-band and a minimum transition region.
R. N. Bates showed several structures of a spur-line filter in a paper, "Design of microstrip spur-line bandstop filters" in Microwaves, Optics and Acoustics, Vol. 1, No. 6, pp. 209-214, November 1977, as shown in FIGS. 2(a) to 2(e). The filter consists of a coupled pair of microstrip lines, a quarter wavelength long (referred to the stop-band center frequency), with an open circuit at the end of one of the coupled lines and both lines connected together at the other end. The original construction was based on using a tuning open stub 7 to adjust the characteristics, as shown in FIG. 2(a). Subsequently, an L-shaped structure was proposed as shown in FIG. 2(b). An inverted L-shaped transmission line section 4, a quarter wavelength long, was placed beside a 50-ohm transmission line 3 to produce resonance. If any signal appearing at the 50-ohm line has a quarter wavelength nearly equal to the L-shaped section 4, the signal resonates with the L-shaped section 4 and cannot be transmitted through. Thus, the L-shaped section serves as a frequency trap like a series tuned circuit. The shorter portion 5 of the L section served as an open stub to tune the circuit. However, this structure did not perform well in terms of selectivity. Then, the structure as shown in FIG. 2(c1) was developed. The feature was that an open, L-shaped groove 6 was etched into a 50-ohm microstrip line. The equivalent circuit is shown in FIG. 2(c2), in which the open circuit section has an equivalent characteristic impedance Z1, the closed circuit section a has an equivalent characteristic impedance Z12, and .theta. is the electrical length. These two characteristic impedances are functions of the line width, dielectric layer thickness, dielectric constant and coupling coefficient. Finally, Bates added mirror structures symmetrically as shown in FIG. 2(d), and in the cascaded structure as shown in FIG. 2(e), similar to a double tuned circuit to increase the selectivity and broadend the stop-band width. As pointed out in his paper, the FIG. 2(e) structure yielded better results. One can see that the resultant characteristics, as shown in FIG. 3(a) had a pass-band loss of -1.5 dB to -2 dB, but a stop-band of only 1 GHz, which is not wide enough. The last two spur-line filters evidently utilized two sections of symmetrical, uniform open-circuit stubs to adjust for the required characteristic.
Hiroshi Saka et.al. also suggested another spur-line filter structure, as described in the paper, "A 12 GHz Very Small Low-Noise Converter Using InGaAs HEMT Monolithic MIC Technology", in The 3rd Asia-Pacific Microwave Conference Proceedings, Tokyo, 1990, pp.677-680. They further added two more sections to the cascaded structure with cascaded, symmetric and uniform open-circuit matching stubs 10, 11 as shown in FIG. 2(f) to effectively couple four tuned circuits together. Otherwise, the structure is similar to Bates'. The resultant characteristics are shown in FIG. 3(b). It can be seen that the pass-band loss is reduced to -1 dB to -2 dB, and the stop-band is widened to approximately 3 GHz.
The two foregoing types of spur-line filters have certain features in common. They all used open-circuit matching sections, having uniform, symmetric structures, i.e. they are all tuned to the same resonant frequency. Such structure limits the matching space, and, thus, cannot yield the optimum design.